(1) Field of the Invention
The present invention relates to an image processing apparatus for processing an image signal.
(2) Related Background Art
As a conventional method of pulse-width modulating (PWM-modulating) an input multi-value signal, a method of comparing the input signal with a triangular wave or saw-tooth wave having a predetermined period is known. A method of operating a counter at a frequency 10 to 1,000 times a sync clock of an input to generate a pulse signal in the counting state is also known. As a method of varying a pulse generation period, as shown in FIG. 23, triangular generators 230 to 232 corresponding to pulse generation periods are arranged, and triangular waves generated by the generators 230 to 232 are compared with an analog video signal 233 by comparators 234 to 236, respectively, thus forming pulse-width modulation (PWM) signals. One of these PWM signals is selected by a selector 237 in correspondence with the value of screen data 238 in units of input pixel periods, and the selected signal is output as a PWM signal.
FIGS. 24A and 24B show examples of such a PWM signal. FIG. 24A shows an example wherein the video data 233 is PWM-modulated by a 200 lines triangular wave 240, and FIG. 24B shows an example wherein the video data 233 is PWM-modulated by a 133 lines triangular wave 241.
The method of PWM-modulating an image signal while switching the period of a triangular wave is particularly effective when a laser beam is flashing-controlled by the PWM signal, and image formation is performed using an electrophotographic technique. More specifically, if the generation period of a triangular wave is prolonged, the gradation property can be improved, and if it is shortened, resolution can be improved. Therefore, the triangular wave is selectively used in accordance with the type of image data (character or photograph data (half-tone data)), thus improving reproducibility of an image. Recently, for a technique of reducing moire noise due to frequency components of the triangular wave and the image signal, demand has arisen for ability arbitrarily to set the shape of a triangular wave like a triangular wave or saw-tooth wave in units of generation periods of the triangular wave. Some approaches are made to correct, in accordance with the shape of a triangular wave or saw-tooth wave, gradation deformation inherent to electrophotography in which a high-light portion of an image tends to be omitted or a shadow portion tends to be flattened.
However, in the above related art, the number of pulse width generation periods to be selected is limited to the number of triangular or saw-teeth generators. In order to realize an arbitrary number of pulse width generation periods, pulse generators corresponding to the arbitrary number must be prepared. The shape of a reference signal such as a triangular or saw-tooth wave cannot be defined, unless, a triangular wave generating means or saw-tooth wave generating means is prepared. A technique of changing a waveform such as a triangular wave must be performed by a nonlinear amplifier such as a log amplifier. Reference signal generating means having different nonlinear characteristics must be prepared in accordance with the shapes of waveform. When an effect equivalent to a triangular wave is to be obtained by a digital signal using a counter or the like, up- and down-count operations must be switched at high speed.
The above-mentioned gradation processed is disclosed on U.S. Pat. applications Ser. Nos. 090,238 (filed on Aug. 25, 1987), 282,896 (a continuation of Ser. No. 013,629, filed Feb. 11, 1987), 425,445 ( a continuation of Ser. No. 897,053, filed Aug. 15, 1986), and 188,712 (filed on Apr. 29, 1988) and U.S. Pat. Nos. 4,661,859, 4,763,199, 4,782,398, 4,800,442, 4,811,037, 4,819,066, 4,831,392, 4,847,654, 4,847,695, 4,864,419, 4,868,684, 4,870,499, 4,873,428, 4,897,734, and 4,905,023, and Japanese Pat. application Laid-Open No. 50-17919, and the like. However, a further improvement is required.